1. Field of the Invention
This invention relates to cylinder and piston constructions for compressors or vacuum pumps, and in particular for oilless compressors or vacuum pumps.
2. Discussion of the Prior Art
Reciprocating piston type compressors and vacuum pumps are of course well known. In such compressors and vacuum pumps, a piston is reciprocated in a cylinder to compress a gas in a compressor or draw a vacuum in a vacuum pump. Since the outer surfaces of the piston slide against the inner surfaces of the cylinder, wear inevitably occurs.
Such wear has an adverse effect on the efficiency of the pump and may even result in damage to the pump. For example, the piston usually has a sealing ring which forms a sliding seal with the walls of the compression chamber. As wear occurs, the integrity of the seal diminishes such that the pressure which was once attainable in the compression chamber no longer can be reached. This is particularly a problem for higher pressure compressors and vacuum pumps, since in such compressors and vacuum pumps the sealing ring is called upon to maintain a higher pressure differential across it.
However, even in low pressure pumps, the seal can be adversely affected by wear to the extent that the pump is no longer effective. In extremely aggravated cases, the wear between the piston and cylinder may become so severe that the piston becomes skewed in the cylinder and may actually impact against parts of the pump such as the valve plate which closely overlies the piston when the piston is at its top dead center position.
Where the application allows it, wear between the piston and cylinder can be materially reduced by the use of a liquid or powder lubricant, hereinafter referred to as a free lubricant, between the sliding surfaces of the piston and cylinder. However, in some applications such as medical and food related applications, the use of free lubricants is not allowed, since even small traces of the lubricant in the pumped gas are undesirable. For example, in Europe it is common for a pub to charge its beer kegs with nitrogen which is pumped to a relatively high pressure. In these applications, an oilless compressor or vacuum pump may be employed, in which no free lubricants are used on the sliding surfaces between the piston and the cylinder, but lubricious materials are employed on those surfaces in an attempt to lessen the wear therebetween.